Is nature natural?

The Large Hadron Collider is starting its 2012 data-taking campaign. While eagerly awaiting this, we're still publishing data and digesting what we have learned from the past two years. And that seemingly tautological question is looming larger

We know that around LHC energies special things happen in physics. The force carriers of the weak force - W and Z bosons - have masses in this energy range. This is why we are sure that if the Standard Model Higgs boson exists, it will be visible (eventually) here too1.

More general arguments based on "naturalness" are often used to argue that other new physics should show up here too. Naturalness, in this context, is the assumption that the parameters in a theory should be about unity, and should not have to be fantastically fine-tuned in order to make the theory work.

In the Standard Model of particle physics, the Higgs mass unfortunately does have to be fantastically fine-tuned. As I wrote here, this is one of the problems supersymmetry could solve. In that case, we ought to see some evidence for it.

Many physicists are getting puzzled by the absence of supersymmetry, or in fact of any new physics which could do its job, in the LHC data. It's too soon to give up on the idea of naturalness, but things have gone far enough that at the end of the SEARCH meeting, several eminent theorists advised people to work on understanding the strong nuclear force (QCD). Partly because it is interesting but mainly because if we can understand it better we can search more effectively for new physics which might be hiding in the data at the LHC.

This of course is what lots of us are doing already. For example, ATLAS just released the first measurements of some new jet substructure variables which will be useful for searching for new particles at the LHC. Some of these were the subject of my first ever blog. The conclusion of the new ATLAS paper is that we do understand QCD (and our experiment) well enough to use these variables; some of the ideas have even been used already in the CMS Higgs search. Such things will be even more useful in the new data, which should start coming this week at a higher collision energy than last year.

That paper is close to my heart because I've worked on those studies myself, but many similar precise measurements, and calculations, need to be done before we can learn all the LHC can tell us. If in the end we do not find supersymmetry, or something similar, the idea of naturalness will be in real trouble.